It is a requirement in an aircraft to supply cool air to a pressurised cabin thereof. Usually the cabin air is recirculated with some of the air being refreshed by air obtained from the exterior of the aircraft.
Typically such external air is bled from an engine of the aircraft and is thus pressurised. Such air requires filtering and cooling before being mixed with recirculating air for introduction into the aircraft cabin for breathing and for the maintenance of comfortable conditions.
It is known to cool the air by exchanging heat in the hot pressurised air with cooler ambient air. This may efficiently be achieved by first compressing the bled air to increase its temperature and pressure before cooling the air. Also it is known to cool the air further by expanding the air over an expansion turbine, which also results in some pressure loss in the compressed air.
With conventional systems, hot cabin air is ejected from the air conditioning system and loss of volume is made up by the cooled conditioned bleed air. Thus the heat energy of the hot cabin air which is ejected, is wasted.
Especially where an aircraft is parked on the ground in hot climatic conditions, the cabin air may become very hot and using a conventional air conditioning system, it may take some time after the air conditioning system is started up, e.g. when the aircraft engine or engines are started, or when a ground based support unit is used to provide air to the air conditioning system, for the cabin air to be cooled to a desirable temperature. Thus a considerable amount of heat energy may be wasted.
Avionic systems such as electronic, hydraulic etc. systems produce heat in use, and at least some such systems, particularly electronic systems, require cooling to protect the system components from heat damage. Again heat energy is discarded and thus wasted by conventional avionics cooling systems.